1. Field of the Invention
The present invention relates to a SAW (surface acoustic wave) filter manufactured by using a GaN piezoelectric thin film, and a manufacturing method therefor. More specifically, the present invention relates to a SAW filter manufactured by using a GaN piezoelectric thin film, and a manufacturing method therefor, in which the filter is obtained by forming a GaN thin film of [0001] direction on a single crystal substrate, and in which the characteristics are superior.
2. Description of the Prior Art
Recently, countries are progressing toward information societies, and in accordance with this trend, the demands for mobile communication are steeply increasing, because it is not subjected to the time limitation and the spatial limitation. If the mobile communication apparatus is to be ensured of its stable operation, not only the system development is important, but also there has to be developed a high performance filter which shows a high sidelobe decay and a frequency stability at high frequency regions.
Further, it is the current trend that the bulk of the mobile communication apparatus becomes smaller, and its components are also provided in the form of a reduced package. Accordingly, in the case of the filter also, the LC filter of the past has been replaced with the dielectric filter, and the dielectric filter is again being replaced with the SAW (surface acoustic wave) filter.
The SAW filter, which is calling attention as the crucial component of the mobile communication apparatus, is gaining its importance as a new component for the image system such as TV broadcasting, HDTV, CATV and the like. Further, in accordance with the expansion in the demand for the mobile communication, the development of a high frequency device of the order of over GHz is being required. To meet this demand, there is being developed the SAW filter which is efficiently operable at a high frequency.
In the research for the development of the GHz band SAW filter, there are required a material having the SAW propagation velocity, a submicron process technology, and a development of a harmonic mode SAW filter.
In manufacturing the SAW filter for utilizing the advantages of the LSAW (Leaky SAW) which has a propagation velocity and an electromechanical coupling factor which are much higher compared with the Rayleigh waves, the following techniques have been known. That is, there is used a substrate which is composed of a piezoelectric single crystal such as 36xc2x0 LiTaO3, 41xc2x0 LiNbO3, 64xc2x0 LiNbO3, LST quartz or the like. In these conventional SAW filters which use LiNbO3, LiTaO3, quartz or the like, the application fields are different depending on the raw material used. That is, in the case of the LiNbO3 single crystal, the k2 value of the electromechanical coupling factor is large, and therefore, it is applied to the wide band width filters. However, it has a disadvantage such that the thermal stability is low. In the case of quartz, although the temperature stability is superior, the propagation velocity and the k2 value of the electromechanical coupling factor are low compared with the LiNbO3 and LiTaO3 single crystals. In the case of LiTaO3, the electromechanical coupling factor is higher than quartz, and the temperature stability is superior compared with LiNbO3, thereby making it possible to compensate the disadvantages of LiNbO3.
However, in the case of the SAW filters using the above described piezoelectric single crystals, the decay constant is high, with the result that the propagation loss is large.
Generally, in the SAW filter, it is required that the k2 value of the electromechanical coupling factor be high, and the temperature stability be superior. In the currently used piezoelectric devices, however, due to the fact that they are mostly made of a single crystal, if the crystal face and the direction for the propagation of the surface acoustic wave are decided, then the propagation velocity and the k2 value of the electromechanical coupling factor become almost constant.
Accordingly, when the propagation velocity and the k2 value are constant, the resonance frequency is decided by the inter-electrode distance of the IDT (interdigital transducer) electrodes. Therefore, if the frequency is to be raised, the electrode pattern width has to be reduced, and therefore, a technical limitation is exposed at frequencies higher than 2 GHz. Particularly, in the case of the duplexer for the PCS, the electrode width has to be reduced to about 0.35 xcexcm, and therefore, it is difficult to satisfy the maximum input power characteristics which are required at the Tx terminal of the duplexer. Further, in the case where the SAW filter of single crystal is applied to an RFIC, it is difficult to form it in a single chip. Therefore, it has to be formed in the hybrid type, with the result that the manufacturing process becomes complicated, and that a high density becomes difficult.
The present invention is intended to overcome the above described problems of the conventional techniques.
Therefore it is an object of the present invention to provide a SAW filter of a high frequency band, in which a GaN thin film is formed to the optimum thickness on an xcex1-Al2O3 single crystal substrate, thereby obtaining superior piezoelectric characteristics and superior input/output characteristics.
It is another object of the present invention to provide a method for manufacturing a SAW filter of a high frequency band, in which a GaN thin film is formed to the optimum thickness and at certain heat treatment conditions on an xcex1-Al2O3 single crystal substrate.
The conditions for manufacturing the SAW filter of the present invention can be derived from the following formula:
 less than Formula greater than 
fo=Vo/2L 
where fo is the resonance frequency, Vo is the SAW propagation velocity, and L is the IDT inter-electrode distance.
That is, as can be seen in the above formula, if a SAW filter of a high resonance frequency is to be manufactured, either the SAW propagation velocity has to be high, or the IDT inter-electrode distance has to be short. However, the SAW propagation velocity is determined only by the inherent properties of the piezoelectric single crystal thin film. Further, as described above, the technology of reducing the IDT inter-electrode distance encounters a technical limitation in manufacturing the SAW filter of over 2 GHz.
Therefore, the present inventor carried out very much research to find out a piezoelectric single crystal device which can overcome the above described limitations. As a result, the following fact was found. That is, if a GaN single crystal thin film is formed in the optimum thickness on an xcex1-Al2O3 single crystal substrate, a SAW carrier velocity was 1.5 times as fast as that of the conventional LiNbO3 and LiTaO3 single crystal substrate. Therefore, even at the same IDT inter-electrode width, a resonance frequency of higher band can be obtained.
Therefore, in one aspect of the present invention, the SAW filter of a high frequency band according to the present invention includes: an xcex1-Al2O3 single crystal substrate; a GaN piezoelectric single crystal thin film of [0001] direction formed to a thickness of 0.3-300 xcexcm on the substrate; and an IDT electrode pattern formed on the GaN piezoelectric single crystal thin film.
In another aspect of the present invention, the method for manufacturing a SAW filter of a high frequency band according to the present invention includes the steps of: preparing an xcex1-Al2O3 single crystal substrate; epitaxially growing a GaN piezoelectric single crystal thin film of [0001] direction to a thickness of 0.3-300 xcexcm on the substrate; and forming an IDT electrode pattern on the GaN piezoelectric single crystal thin film.